When handling volatile liquids such as hydrocarbons including gasoline and kerosene, air-volatile liquid vapor mixtures are readily produced. The venting of such air-vapor mixtures directly into the atmosphere results in significant pollution of the environment and a fire or explosion hazard. Accordingly, existing environmental regulations require the control of such emissions.
As a consequence, a number of processes and apparatus have been developed and utilized to recover volatile liquids from air-volatile liquid vapor mixtures. Generally, the removed volatile liquids are liquified and recombined with the volatile liquid from which they were vaporized thereby making the recovery process more economical.
The initial vapor recovery systems utilized in the United States in the late 1920's and early 1930's incorporated a process combining compression and condensation. Such systems were originally only utilized on gasoline storage tanks. It wasn't until the 1950's that local air pollution regulations began to be adopted forcing the installation of vapor recovery systems at truck loading terminals. Shortly thereafter, the "clean air" legislation activity of the 1960's , which culminated in the Clean Air Act of 1968, further focused nationwide attention on the gasoline vapor recovery problem. As a result a lean oil/absorption system was developed. This system dominated the marketplace for a short time.
Subsequently, in the late 1960's and early 1970's cryogenic refrigeration systems began gaining market acceptance (note, for example, U.S. Pat. No. 3,266,262 to Moragne). While reliable, cryogenic systems suffer from a number of shortcomings including high horsepower requirements. Further, such systems require relatively rigorous and expensive maintenance to function properly. Mechanical refrigeration systems also have practical limits with respect to the amount of cold that may be delivered, accordingly, the efficiency and capacity of such systems is limited. In contrast, liquid nitrogen cooling systems provide more cooling than is required and are prohibitively expensive to operate for this type of application.
As a result of these cryogenic refrigeration system shortcomings, alternative technology was sought and adsorption/absorption vapor recovery systems were more recently developed. One such system is disclosed in, for example, U.S. Pat. No. 4,066,423 to McGill et al. Such systems utilize a bed of solid adsorbent selected, for example, from silica gel, certain forms of porous mineral such as alumina and magnesia, and most preferably activated charcoal or carbon. These adsorbents have an affinity for volatile hydrocarbon liquids. Thus, as the air-hydrocarbon vapor mixture is passed through the bed, a major portion of the hydrocarbons contained in the mixture are adsorbed on the bed. The resulting residue gas stream comprising substantially hydrocarbon-free air is well within regulated allowable emission levels and is exhausted into the environment.
During the adsorption process, the adsorbent increases significantly in temperature. This is due to the release of the heat of adsorption of hydrocarbon and also side exothermic reactions with impurities contained in the air-hydrocarbon vapor mixture being processed. As a result of these factors, undesired and potentially unsafe overheating may occur under certain operating conditions. In order to better prevent such overheating of the beds of adsorbent, it is well known to establish a residual heel of hydrocarbons in the adsorbent prior to conducting vapor recovery processing.
In the past, this "preconditioning" of the adsorbent has been completed "on-site" in the actual vapor recovery system. Specifically, new/clean adsorbent is charged into the reaction vessel. The reaction vessel is then sealed and a vacuum is established. Nitrogen is then delivered to the reaction vessel to provide an inert atmosphere and gasoline vapor and nitrogen are then circulated through the reaction vessel and, therefore, the adsorbent in the bed of the reaction vessel for an extended period of time. During this gasoline vapor-nitrogen circulation, the adsorbent heats up due to the release of the heat of adsorption. Once the desired heel is established, however, the adsorbent normally cools down to near ambient temperature. After the establishment of the residual heel is verified, the vapor recovery system is ready to be returned to normal field operation.
It, of course, should be appreciated that as this preconditioning process is performed the vapor recovery system is out of service. Accordingly, the system is unable to provide any emission control and, therefore, the terminal loading operation is also out of service. Due to the difficulty and uncertainty of preconditioning adsorbent, the preconditioning process generally takes at least 30 hours to complete and may even take up to one hundred hours in extreme conditions. As large loading terminal operations may generate revenue of up to $3 million per 24 hour period, it should be appreciated that any shut down for the preconditioning of the adsorbent leads to a substantial loss of revenue. Further, the rerouting or rescheduling of terminal loading activity through other terminal locations is troublesome, time consuming and costly. A need, therefore, is clearly identified for an improved method for changing the adsorbent in the reaction vessel beds of a vapor recovery system that significantly reduces the downtime of the vapor recovery system and, therefore, the downtime of the loading terminal.